The electronic components of an integrated circuit chip, in order to operate properly, need to be supplied with a constant power voltage as defined by a specified tolerance range (e.g., 5%). Available regulated power supplies for integrated circuits may readily meet such a tolerance at DC frequencies. The components of a typical integrated circuit, however, include high-speed switches that transiently draw current at very high frequencies. As the operating frequency increases, the output impedance of the power delivery system increases due to inductance in the system, contributed primarily by conductors that connect the chip to a package structure in a completed integrated circuit assembly. Such increased output impedance can cause the voltage supplied to the chip to drop below tolerance. Decoupling capacitors, or decaps, may be added to the system in parallel with the inductance in order to reduce the output impedance. The decoupling capacitance added to the chip lowers the output impedance of the power delivery system at high frequencies because capacitor impedance is inversely proportional to frequency. Decaps, usually located on the chip near the current drawing components, store charge and give energy back to the chip components as needed which tends to hold the power supply voltage constant during high frequency operation. With decoupling capacitors, it is possible to make a low impedance power delivery system that meets a specified target impedance up to very high frequencies (e.g., several hundred Mhz).
As noted above, the power delivery system possesses both inductance, mainly due to the package connections, and capacitance, due to decaps as well as the inherent capacitance on the chip due to various components and structures. The inductance of the package, however, forms a parallel RLC circuit with the capacitance of the chip that resonates at the frequency f=½π(LC)½, where L is the equivalent series inductance of the system and C is the total capacitance on the chip between the voltage and ground nodes. The impedance of an inductance in parallel with a capacitance is maximized at the resonance frequency. At that frequency, the chip components therefore see a high output impedance from the power delivery system, usually much higher than the target impedance. The capacitance on the chip is not low enough in impedance and does not store enough charge to deliver the current needed by the chip components at the resonance frequency. The electronic components of the chip may then be starved for current, and the power supply voltage supplied to the chip can drop out of the specified tolerance range.